Electrical discharge device



Feb. 21, 1939. E ER ER 2,148,017

ELECTRICAL DICHARGE DEVICE Filed Feb. 19, 1957 Edmund Ger/var INVENTOR Patented Feb. 21, 1939 PATENT OFFICE ELECTRICAL DISCHARGE DEVICE Edmund Gcrmer, Berlin-Wannsee, Germany Application February 19, 1937,Serial No.126,600 2 Claims. (Cl. 176-124) This invention relates to electric discharge lamps of the type in which the discharge passes through a gaseous atmosphere. More particularly the invention relates to lamps of this type in which the gaseous atmosphere includes the vapor of a metal heated by the discharge to a temperature considerably exceeding atmospheric.

Lamps of this general type have been known and in common use prior to my invention. In such lamps the hottest portion of the tubular envelope in which the gaseous atmosphere is ordinarily confined usually between the electrodes because this part is occupied by the discharge. Nevertheless, since the vapor pressure of the metal vapor. e. g., sodium, may be lessthan that of the permanent gas there is apt-to remain even on the portion of the tube between the electrodes patches of unvaporized metal which partially obscure the light and thereby reduce the efiiclency of the lamp.

It is an object of the present invention to remove this defect and to provide lamps in which the wall of the tube between the electrodes is maintained at a temperature sufficiently high to avoid condensation of the metal thereon.

The present application is a continuation in part of my prior application, Serial No. 500,346, filed December 5, 1930. In that application is shown and described a metal vapor lamp of the general type referred to above in which the electrode chambers are of larger diameter than the arc tube so that any condensation which occurs will occur behind the electrode and any metal which exists within the arc tube when the arc is started will be vaporized by heat of the arc. In

the preferred example described in my said prior application mercury was utilized and advantage was taken of its surface tension to return the condensate rapidly to the bottom of the arc tube thus maintaining a constant distillation circuit and thus sweeping to the ends of the tube any deposits sputtered or evaporated from the electrode and keeping the arc portion of the tube free from any such deposit.

With other metals, e. g., sodium or thallium or gailium especially referred to in my said prior application, however, such return does not occur and the unvaporized portion of such metals collects during operation in the cooler electrode chambers.

In either case the walls of the arc tube proper, and. excepting oniy, in the case of the mercury lamp, the small pool to which the condensate returns when the constant distillation method is utilized, are kept clear of any metal deposit and the full efliciency of the tube is, therefore, maintained. Although, in my prior application this was described particularly in connection with high pressure lamps, I have found it to be an im: portant advantage with both high and low pressure lamps in which the current loading is suflicient to maintain the operating temperature of the arc tube proper at a temperature well above atmospheric temperature so that a substantial temperature difierential can be maintained between the electrode chambers and the arc tube proper.

According to the preferred embodiment of my invention the desired temperature differential is produced by making the tube between the electrodes of smaller cross-sectional area than in the space near or behind the electrodes. It will be understood, however, that this result can be accomplished also by means of shields or baflies, e. g., as illustrated in my said prior application, Serial No. 500,346, which may to some extent intercept the radiation and convection of heat to a limited area on the wall of the tube near or behind the electrode where the condensate is to occur.

In either case the cross-sectional area of the arc tube between the electrodes is sufliciently small that during operation the wall of the tube is heated by the discharge so hot that the metal vapors cannot condense thereon and any metal vapor which may have deposited thereon will be distilled away toward the end of the tube.

It must be observed, however, that with the tube thus. constructed the vapor pressure of the vaporizable metal is determined by the temperature of the cool end toward which the sodium distills.

Thus the proportions and thermal insulation and protection against external draughts, etc., become more important in this type of tube than in those necessary to maintain the exterior of the end at a higher temperature than would be necessary if the metal were permitted to remain in the middle of the tube. This may be attained by various means, such as enclosing the ends in thermal insulatingmaterial or coating them with heat reflecting layers or providing electrical resistance heaters near the ends, which may in part ballast the discharge.

The constricted portion of the envelope may have many forms. Thus the envelope may have many forms. Thus the envelope may have the form of two truncated cones intersecting their narrowest section midway between the electrodes so that there is at this point a single narrowest cross-section, or there may be a relatively narrow substantially cylindrical portion intervening between two conical or bulbous ends. In this latter case the cylindrical tubular portion may with admum diameter of the envelope.

In the accompanying drawing I have shown two embodiments of the invention designed particularly for sodium lamps in which a permanent gas, e. g., argon or neon or other rare gases, and especially mixtures of them, at pressures, e. g., of the order of 1 to 20 mm. and a quantity of sodium are contained within a suitable transparent tube I. The electrodes 2 and 3 which are advantageously of the solid self-heating type described, ior example, in my said prior application, are sealed into the tube near its ends.

As shown in the drawing the center portion of the tube between the electrodes is of diminished cross-sectional area and the widest parts of the tube are close to the electrodes. This shape and the dimensions of the lamp are carefully designed and proportioned with respect to the energy loading for which the lamp is intended, so that during normal operation the cooler parts of the tube i close to the electrodes 2 and 3 will be sufliciently heated to maintain a pressure of the sodium of about the same order or somewhat less than the pressure of the permanent gas.

In the operation of these tubes, as soon as the discharge is established between the electrodes 2 and 3 and is loaded suflicientlyto heat the wall of the tube between the electrodes! and 3 above the temperature of the saturated vapor or the pressure existing within the tube, any metal theretoforedeposited on thewalls of this portion of the tube I will be evaporated; and, if such deposit is in excess of that required to establish the desired operating pressure, the excess will thereupon be transferred by distillation to the cooler ends of the tube around and/or behind the electrodes 2 and 3.

It is to be understood that the particular forms of tubes shown in Figs, 1 and 2 or the drawing are not essential to my invention and that the desired diflferential can be obtained in other ways,

' as for example, by providing cooling fins or external chambers more readily cooled by radiation or convection currents, e. g., by conduction of heat along the in-lead wire to cool the condensation chamber, or as illustrated in the copending application of Hans J. Spanner, Serial No. 107,190, filed October- 23, 1936. Furthermore, as there described, the vapor pressure within the lamp may be controlled by a resistance heater associated with the condensation chamber controlled by the electrical characteristics 01' the discharge. Whether the condensation chamber is merely an enlarged diameter portion of the tube or is an area behind a baflie or shield or is an extension as suggested or any other type of condensation space, it is essential that the temperature diflerential should exist between the limited condensation area and the arc portion of the tube; and to this end it is, therefore, important that the crosssectional area of the tube between the electrodes should be sufilciently small with reference to the heat production in the discharge and the heat dissipating capacity of the tube so that, with the normal operating load, its temperature will be raised throughout above the condensation temperature (dew point) for the metal vapor used.

Although I have shown electrodes heated by the action of the discharge itself, it will be understood, of course, that my invention is not limited to any particular type of electrodes and that filamentary resistance heated electrodes and also uni-potential indirectly heated electrodes can also be used.

What I claim is:

1. A metal vapor discharge device which comprises a sealed tube, electrodes positioned at opposite ends thereof, electrode support wires sealed through the tube, and a filling in said tube comprising an excess of. a vaporizable metal, said tube having a tubular portion between the electrodes close to the discharge path and enlarged electrode chambers having smoothly rounded ends whereby to avoid pockets in which the metal would collect, and an energizing circuit adapted to load the discharge between said electrodes with a current sufiicient to heat said tubular portion above the dew point of said vaporizable metal, and said electrode chambers having a heat dissipating capacity such that their walls remain approximately at the dew point of said metal when the discharge is operating in said energizing circuit.

2. A discharge device as described in claim 1, in which the parts of the tube adjoining the electrode chambers taper towards a central tubular portion of smaller diameter so as to provide for smooth circulation of gases within the tube to the end of the electrode chamber, whereby any condensate which remains during operation will be confined to the surface of the electrode chamber behind the electrode.

EDMUND GERMER. 

